To evaluate a possible correlation of susceptibility to hepatic injury and rapid acetylation of isoniazid, we have now genetically phenotyped 26 non-Oriental patients from the Public Health Service trial who had recovered from isoniazid hepatitis. To determine whether other differences occurred in the metabolism of isoniazid by rapid and slow acetylators of the drug, radiolabeled isoniazid and acetylisoniazid were given to normal volunteers and urinary metabolites were isolated and identified.